Change BKPT and UDF encodings on ARM.

src/include/arm_sandbox.h

+/*
+ * Copyright (c) 2012 The Native Client Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/*
+ * Minimal ARM sandbox constants.
+ *
+ * These constants are used in C code as well as assembly, hence the use of
+ * the preprocessor.
+ */
+
+#ifndef NATIVE_CLIENT_SRC_INCLUDE_ARM_SANDBOX_H_
+#define NATIVE_CLIENT_SRC_INCLUDE_ARM_SANDBOX_H_ 1
+
+/*
+ * Specially chosen BKPT and UDF instructions that also correspond to
+ * BKPT and UDF when decoded as Thumb instructions.
+ * All other BKPT/UDF values are disallowed by the validator out of paranoia.
+ */
+
+/*
+ * BKPT #0x5BE0: literal pool head.
+ *
+ * Treated as a roadblock by the validator: all words that follow it in
+ * a bundle aren't validated and can't be branched to.
+ */
+#define NACL_INSTR_LITERAL_POOL_HEAD 0xE125BE70
+
+/*
+ * NACL_INSTR_BREAKPOINT, NACL_INSTR_HALT_FILL and NACL_INSTR_ABORT_NOW
+ * are intended to be equivalent from a security point of view. We
+ * provide the distinction between them just for debugging
+ * purposes. They might also generate different POSIX signals. In
+ * principle it should be safe for a debugger to skip past one of these
+ * (unlike NACL_INSTR_POOL_HEAD), because the validator validates the
+ * instructions that follow.
+ */
+
+/*
+ * BKPT #0x5BEF: generic breakpoint.
+ *
+ * Usable statically by users or dynamically by the runtime.
+ */
+#define NACL_INSTR_BREAKPOINT        0xE125BE7F
+
+/*
+ * UDF #0xEDEF: halt-fill.
+ *
+ * Generated at load time.
+ */
+#define NACL_INSTR_HALT_FILL         0xE7FEDEFF
+
+/*
+ * UDF #0xEDE0: abort-now.
+ *
+ * Required by some language constructs such as __builtin_trap.
+ */
+#define NACL_INSTR_ABORT_NOW         0xE7FEDEF0
+
+/*
+ * UDF #0xEDE1: always fail validation.
+ *
+ * It's guaranteed to always fail, and can be used to initialize buffers
+ * that are expected to be filled later.
+ */
+#define NACL_INSTR_FAIL_VALIDATION   0xE7FEDEF1
+
+#endif  /* NATIVE_CLIENT_SRC_INCLUDE_ARM_SANDBOX_H_ */